In general, this invention relates to electronic timers of the type that produce command signals to program the turning on and off of external devices. More particularly, it relates to an electronic timer that is so organized as to adapt it for a modular arrangement.
Various types of timers have heretofore been developed to provide for programming switching operations. Some such timers are electromechanical in nature and include a bulky motor-driven disk that rotates to cause peripheral switches to open and close at selected angular orientations of the disk. Other such timers are electronic in nature.
A well known approach to organizing such an electronic timer involves an electronic store, the stored value of which is compared with the current value defined by an electronic digital clock. To provide a separate comparator circuit for each different switching function involves a substantial amount of circuitry in circumstances in which a multiplicity of switching functions are to be programmed.
In view of the foregoing disadvantage, it has heretofore been proposed to provide a multiplexer that controls a sampling operation whereby seriatum comparison operations are carried out by a time-shared comparator circuit.
As for the multiplexing approach, it should be noted that a disadvantage inheres in this approach with respect to the precision with which start and stop times can be defined in a system for producing a plurality of command signals. In particular, with a single comparator being time shared by a plurality of stores, it is not possible to provide for simultaneously initiating a plurality of command signals. Thus, in circumstances in which for example it is desired to simultaneously start one device and stop another device, this can not be achieved by the multiplexed timer itself. Instead, one command signal must precede the other.